How do you create custom notifications in Swift 3?

Question

In Objective-C, a custom notification is just a plain NSString, but it's not obvious in the WWDC version of Swift 3 just what it should be.

Answer 1

NSNotification.Name(rawValue: "myNotificationName")

Answer 2

You could also use a protocol for this

protocol NotificationName {
    var name: Notification.Name { get }
}

extension RawRepresentable where RawValue == String, Self: NotificationName {
    var name: Notification.Name {
        get {
            return Notification.Name(self.rawValue)
        }
    }
}

And then define your notification names as an enum anywhere you want. For example:

class MyClass {
    enum Notifications: String, NotificationName {
        case myNotification
    }
}

And use it like

NotificationCenter.default.post(name: Notifications.myNotification.name, object: nil)

This way the notification names will be decoupled from the Foundation Notification.Name. And you will only have to modify your protocol in case the implementation for Notification.Name changes.

Answer 3

@CesarVarela's answer is good, but to make the code slightly cleaner, you can do the following:

extension Notification.Name {
    typealias Name = Notification.Name

    static let onSelectedSkin = Name("on-selected-skin")
    static let onFoo = Name("on-foo")
}

Answer 4

I may suggest another option which is similar to what @CesarVarela suggested.

extension Notification.Name {
    static var notificationName: Notification.Name {
        return .init("notificationName")
    }
}

This will let you post and subscribe on notifications easily.

NotificationCenter.default.post(Notification(name: .notificationName))

Hope this will help you.

Answer 5

if you use string-only custom notifications, there's no reason to extend any classes but String

    extension String {
        var notificationName : Notification.Name{
            return Notification.Name.init(self)
        }
    }

Answer 6

Notification.post is defined as:

public func post(name aName: NSNotification.Name, object anObject: AnyObject?)

In Objective-C, the notification name is a plain NSString. In Swift, it's defined as NSNotification.Name.

NSNotification.Name is defined as:

public struct Name : RawRepresentable, Equatable, Hashable, Comparable {
    public init(_ rawValue: String)
    public init(rawValue: String)
}

This is kind of weird, since I would expect it to be an Enum, and not some custom struct with seemingly no more benefit.

There is a typealias in Notification for NSNotification.Name:

public typealias Name = NSNotification.Name

The confusing part is that both Notification and NSNotification exist in Swift

So in order to define your own custom notification, do somethine like:

public class MyClass {
    static let myNotification = Notification.Name("myNotification")
}

Then to call it:

NotificationCenter.default().post(name: MyClass.myNotification, object: self)